Simulating the Fumigation Process using Lagrangian Particle Dispersion Model combined with Large-Eddy Simulation

A Lagrangian particle dispersion model (LPDM) is used to study fumigation of pollutants in and above the entrainment zone into a growing convective boundary layer. Probability density functions of particle location with height and time are calculated from particle trajectories driven by the sum of the resolved scale velocity from a large-eddy simulation (LES) model and the stochastic subgrid scale (SGS) velocity. The crosswind-integrated concentration (CWIC) fields show good agreement with water tank experimental data. Comparison of the LPDM output with an Eulerian diffusion model output based on the same LES flow shows qualitative agreement with each other except that greater overshoot maximum of the ground-level concentration occurs in Eulerian model.

The dimensionless CWICs near the surface for sources located above the entrainment zone collapse to a nearly universal curve provided that the profiles are time shifted, where the shift depends on the source heights, while those for sources located within the entrainment zone show a different behavior. Thus, fumigation from sources above the entrainment zone and within the entrainment zone should be treated separately. Contrary to previous studies, application of Taylor's translation hypothesis to the fumigation process is found to be inappropriate.

Equilibrium concentrations from the LPDM for fast entrainment case exhibit some accumulation of particles near the surface rather than a well-mixed vertical profile, implying that our SGS velocity model needs improvement for rapidly growing boundary layer.